1. Field of the Invention
The present invention relates to a liquid droplet ejection apparatus and an image forming apparatus, and more particularly to a structure of a liquid droplet ejection apparatus that uses an ejection head in which numerous liquid droplet ejection ports (nozzles) are arranged two-dimensionally in high density, and to an image forming apparatus which forms an image on a recording medium using liquid droplets ejected from the liquid droplet ejection apparatus.
2. Description of the Related Art
An inkjet recording apparatus causes ink droplets to be ejected onto a recording medium by ejecting ink from a recording head in accordance with a printing signal while causing recording paper or another recording medium to move relative to the recording head (ejection head) provided with nozzles for ink ejection, thereby forming an image using these ink dots.
Increased nozzle density is desired for enabling photo-quality high-resolution printing, and a technique related to this object is disclosed in Japanese Patent Application Publication No. 2001-334661 which achieves high nozzle density by a configuration in which rectangular chambers (pressure chambers) corresponding to the nozzles are arranged two-dimensionally in a matrix.
However, when the nozzle density is increased using the technique disclosed in Japanese Patent Application Publication No. 2001-334661, and a full-line recording head is structured having rows of nozzles that have a length corresponding to the entire width of the image recordable width, irregularities in the image saturation of the printed result sometimes occur due to differences in the extent of liquid droplet aggregation on the recording medium that occur due to differences in ejection time between adjacent dots. This phenomenon and its causes will be described in general using FIGS. 11 to 13C.
In FIG. 11, the reference numeral 110 indicates the full-line inkjet head (hereinafter referred to as the “head”), and the reference numeral 116 indicates the recording medium (paper, for example). In this arrangement, the recording medium 116 is conveyed from the bottom to the top of FIG. 11 (in the direction of the arrow S). The head 110 has a length corresponding to the entire width W of the recording medium 116, and is fixedly mounted so as to extend along the direction substantially perpendicular to the delivering direction of the recording medium 116.
The head 110 has a structure in which the ink chamber units (liquid droplet ejection elements) 153 made up of the nozzles 151, which are ink droplet ejection ports, the pressure chambers 152 corresponding to the nozzles 151, and other components are arranged (two-dimensionally) in a matrix all along the length corresponding to the entire width W of the recording medium 116. Specifically, a matrix structure is formed in which nozzle rows are formed in the travel direction in the direction perpendicular (the direction of the arrow M; the main scanning direction) to the delivering direction (the direction of the arrow S; the sub-scanning direction) of the recording medium 116, and in an oblique row direction at a certain angle θ not perpendicular to the travel direction. The reference numeral 154 indicates an ink supply port to the pressure chamber 152.
As shown in the magnified view of FIG. 12, when the pitch between nozzles in the row direction having the angle θ with respect to the travel direction (main scanning direction) is “d,” the substantial nozzle pitch P projected so as to align with the main scanning direction becomes d×cos θ.
In a full-line head comprising rows of nozzles that have a length corresponding to the entire width of the image recordable width, the “main scanning” is defined as to print one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) in the width direction of the recording paper (the direction perpendicular to the delivering direction of the recording paper) by driving the nozzles in one of the following ways: (1) simultaneously driving all the nozzles; (2) sequentially driving the nozzles from one side toward the other; and (3) dividing the nozzles into blocks and sequentially driving the blocks of the nozzles from one side toward the other.
In particular, when the nozzles 151 arranged in a matrix are driven, the main scanning according to the above-described (3) is preferred. More specifically, the nozzles 151-11, 151-12, 151-13, 151-14, 151-15 and 151-16 are treated as a block (additionally; the nozzles 151-21, 151-22, . . . , 151-26 are treated as another block; the nozzles 151-31, 151-32, . . . , 151-36 are treated as another block, . . . ); and one line is printed in the width direction of the recording paper 116A by sequentially driving the nozzles 151-11, 151-12, . . . , 151-16 in accordance with the conveyance velocity of the recording paper 116.
On the other hand, the “sub-scanning” is defined as to repeatedly perform printing of one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) formed by the main scanning, while moving the full-line head and the recording paper relatively to each other.
When the nozzle drive described in (3) above is performed by the head 110 having the nozzle arrangement structure shown in FIG. 12, the time interval during which two adjacent dots are deposited in the main scanning direction varies according to the combination of nozzles. In other words, when ejection is driven from one end of the nozzle block in the sequence 151-i1→151-i2→151-i3→ . . . →151-i6 (wherein i is an integer) according to the sequence of (3) above, dots deposited by 151-i1 and 151-m6 (wherein m=i+1) are adjacent to each other on the recording medium 116, but the time interval during which these two dots are deposited differs from and becomes large with respect to the ejection time difference between dots deposited by other nozzles (for example, 151-i1 and 151-i2, 151-i2 and 151-i3, and others).
When the recording medium 116 is conveyed at high velocity, or in the case of a medium that is slow to stabilize, ejection by the adjacent nozzle is performed with the previously ejected ink droplet already on the recording medium 116. Whereupon, the liquid droplets come in contact with each other on the surface of the recording medium 116, and subsequently ejected liquid droplets are pulled toward and coalesce with the already ejected liquid droplets (see FIGS. 13A to 13C).
As shown in FIG. 13A, immediately after landing on the recording medium 116, the previously ejected liquid droplet 190 has a small surface area of contact with the recording medium 116, but the liquid droplet 190 eventually spreads as time elapses, and continues to soak into the recording medium 116 as shown by the dot-dashed line in FIG. 13A.
As shown in FIG. 13B, before the liquid droplet 190 has been completely absorbed into the recording medium 116 (in the state in which the liquid droplet 190 is present on the surface of the recording medium 116), when the subsequent liquid droplet 192 (second droplet) is ejected, these liquid droplets 190 and 192 come in contact with each other on the surface of the recording medium 116, and, as shown in FIG. 13C, the second liquid droplet 192 is pulled toward and coalesces with the first liquid droplet 190. As a result, the coalesced dot 196 is formed in a position displaced to the left of the original dot position 194 indicated by the dashed line in the same figure. At this time, the left side of the coalesced dot 196 is formed larger with respect to the size of the dot when the first liquid droplet 190 is fixed by itself.
The phenomenon described above occurs continuously within the nozzle block. Describing the liquid droplet ejected from the nozzle 151-i6 last in line in the nozzle block, the dot deposited by this nozzle 151-i6 thus comes in contact with the two dots that include the dot deposited by the nozzle 151-i5 and the dot deposited by the nozzle 151-m1 (wherein m=i+1), but since the dot deposited by the nozzle 151-m1 is deposited at an earlier time than the dot deposited by the nozzle 151-i6 (deposited at the same timing as the dot deposited by the nozzle 151-i1), it is fixed sooner. Therefore, the dot deposited by the nozzle 151-i6 is drawn toward the dot deposited by the nozzle 151-i5 immediately to the left thereof, with which the ejection time difference is small.
The results of this droplet deposition are shown in FIGS. 14A and 14B. FIG. 14A is a schematic view showing the positioning of the dots after the fluid has moved due to aggregation of the deposited dots, and FIG. 14B is a schematic view of the results of aggregation of groups of dots in the same row in the paper conveyance direction (sub-scanning direction).
As shown in FIGS. 14A and 14B, the distance PD1 between adjacent dots deposited by the nozzles 151-i6 and 151-m1 becomes larger than the distance PD2 between adjacent dots deposited by the other nozzles 151-i1 through 151-i6, and a portion having a lesser concentration compared to other portions is formed in the position on the recording medium 116 that corresponds to the space between these nozzles 151-i6 and 151-m1. When sub-scanning is performed while the recording medium 116 is conveyed, since the phenomenon described above is repeated in the same manner in the sub-scanning direction, an uneven band of lesser concentration appears in the position corresponding to the space between the nozzles 151-i6 and 151-m1 (see FIG. 14B). The period with which this uneven band is repeated (the spatial repetition cycle) becomes the period of the pitch of one row along the row direction that is slanted at angle θ in the two-dimensional arrangement of the nozzles 151 described using FIGS. 11 and 12 (the distance between nozzles 151-11 and 151-21; specifically, the pitch of the nozzle blocks in the row direction).
In the case of a high-density head which achieves photo-quality high-resolution printing, since this period is about 0.1 mm to 1 mm, it overlaps a period that is easily recognizable to the human eye, and is identified as an undesirable banding artifact.
An example is described above in which an ink droplet deposited on the recording medium is fixed thereon by soaking into the recording medium, but aggregation also occurs in a system in which the ink droplet deposited onto the recording medium is fixed on the recording medium by curing (hardening) or drying. The same drawbacks as in the abovementioned case of fixation by soaking therefore also occur in the case of fixation by curing or drying.